Le français est une langue qui a beaucoup évolué mais les relations de base avec l’anglais sont restées.

In “old” French it was normal to use Passé simple and Passé antérieur as Speaking language but today these tenses are more used as Written Language.

1. If you say Je fus content – I was happy (Passé simple – Simple Past, Today used to write stories for Ex.)

2. It sounds the same as J’étais content – I was happy (Imparfait – Simple Past)

The only difference is the confirmation of 2. Now I am no longer happy

Let’s take your example

1. Je parlai avec ma soeur (passé simple)
2. J’eu parlé avec ma soeur (passé antérieur)
3. J’ai parlé avec ma soeur (passé composé)
4. J’avais parlé avec ma soeur (plus-que-parfait)
5. I spoke to my sister (simple past)

1, 2, 3 and 4 sound like 5 in English because the action is done before the sentence is spoken. But from language to language we have differences in the actions concurrence management, how to know actions order in the story.

To avoid these historical, grammatical, stylistic differences, Google Translator takes the must “speak-able” solution. You, as native speaker of your language and knowing grammar basics of French you should determine which tense is most suitable.

I suppose Google Translator is focusing on the way to speak the language.

The first sentence uses passé composé. This tense has essentially replaced the passé simple in spoken French and is slightly doing the same in written French too.

J’ai parlé avec ma sœur

is then definitely the best translation for

I spoke to my sister.

The passé simple is rare, so rare that Google Translate even thinks it might have been mistyped:

The passé simple at the first person singular (je parlai) is also indistinguishable from the imparfait (je parlais) in spoken French, which doesn’t help either.

On the other hand, the plus-que-parfait is common in written French and is not incongruous in spoken French.

The issue with j’avais parlé à ma soeur is that it lacks context.

Should you give one to Google translate and it selects the Past Perfect:

j’avais parlé avec ma sœur ➔ I spoke to my sister

j’avais déjà parlé avec ma sœur ➔ I had already spoken with my sister

Google Translate is incapable of determining context and should be avoided for anything other than trying to understand the gist of a text.

The passé composé is equivalent to three different English constructions:

1. I spoke to my sister [and she agrees]. (simple past)
2. I have spoken to my sister [three times in the last year]. (present perfect)
3. I did speak to my sister [but she refuses to reconsider] (emphatic past)

GT has no way of knowing which of these is correct, so it just chooses the top one. That shouldn’t have any bearing on which one you select as the correct translation given the context.